Predicting the occurrence of iron chlorosis in grapevine with tests based on soil iron forms

Aims: Iron deficiency symptoms (leaf chlorosis and depressed growth) are common in grapevine growing on calcareous soils. The objective of this study was to investigate the relationships between these symptoms and the properties of Spanish vineyard soils. Methods and results: Soils from thirty vineyards with leaf chlorosis were used to pot '110 Richter' rootstock for three seasons. Mean leaf chlorophyll concentration, as measured via SPAD, was positively correlated with the soil content in poorly crystalline Fe oxides, as estimated by extraction with various reagents, and negatively correlated with the contents in calcium carbonate equivalent and active lime. Iron deficiency affected plant growth but none of the measured growth variables was correlated with soil properties. The relationships between SPAD and acid ammonium oxalate-, citrate/ascorbate-and unbuffered hydroxylamine-extractable Fe conformed to the linear-plateau model, from which clear-cut critical levels (345, 425 and 8 mg kg-1, respectively) could be established. This was not possible with tests based on the alkalinity properties (e.g. the calcite ion activity product), the IPC (« Indice du pouvoir chlorosant »), and the DTPA-extractable Fe test, which generally exhibited a poor predictive value. Conclusion: Tests based on reagents capable of extracting, in part or quantitatively, the most reactive forms of soil Fe were useful to estimate the risk of Fe chlorosis in grapevine. Acid ammonium oxalate, citrate/ascorbate and unbuffered hydroxylamine were the best extractants for this purpose. Significance and impact of study: This study has shown the limited usefulness of tests based on the contents and reactivity of the soil carbonate to predict the occurrence of Fe chlorosis in grapevine; tests capable of estimating the contents of the labile soil Fe forms constitute the best alternative.